U.S. patent number 4,292,685 [Application Number 05/911,954] was granted by the patent office on 1981-09-29 for apparatus and method for controlling crosspolarization of signals in a frequency reuse system.
Invention is credited to Lin-Shan Lee.
United States Patent |
4,292,685 |
Lee |
September 29, 1981 |
**Please see images for:
( Certificate of Correction ) ** |
Apparatus and method for controlling crosspolarization of signals
in a frequency reuse system
Abstract
Apparatus and method for controlling the crosspolarization of
signals in a satellite communications, frequency reuse system. Each
earth station in the system includes one crosscoupling network that
compensates for the crosspolarization of signals transmitted by
such earth station uplink to the satellite due to the propagative
medium around such earth station, and an adaptive feedback control
system having another crosscoupling network that compensates for
the crosspolarization of signals received from the other earth
stations due to such propagative medium around such earth
station.
Inventors: |
Lee; Lin-Shan (Stanford,
CA) |
Family
ID: |
25431162 |
Appl.
No.: |
05/911,954 |
Filed: |
May 31, 1978 |
Current U.S.
Class: |
455/13.2;
342/358; 342/361; 342/365; 370/201; 370/317; 455/295; 455/304;
455/60; 455/63.1 |
Current CPC
Class: |
H04B
7/002 (20130101) |
Current International
Class: |
H04B
7/00 (20060101); H04B 007/155 (); H04B 007/185 ();
H04B 015/00 () |
Field of
Search: |
;325/60,65,62,63,367,371,472,476,477,1,3,4,56 ;179/15BP,15AN,15BC
;333/18,28R,7T,21A ;328/162,165,167 ;343/1PE,200,1ST
;455/11,12,52,59,63,71,295,303-306 ;370/6 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Cooper, C. P., "Methods of Adaptive Cancellation for Dual
Polarization Satellite Systems," Marconi Review, vol. 39, No. 200,
pp. 1-24, First Quarter 1976..
|
Primary Examiner: Bookbinder; Marc E.
Attorney, Agent or Firm: Philips, Moore, Weissenberger,
Lempio & Majestic
Claims
What is claimed is:
1. Apparatus for controlling the crosspolarization between dual
reference signals propagated in a frequency reuse satellite
communications system having a plurality of stations communicating
via a satellite, in which the dual reference signals transmitted by
each station are of different polarization, and the propagative
medium around each station causes the crosspolarization of the dual
reference signals transmitted and received by each station, the
apparatus being for use at each station, comprising:
(a) first means for controllably crosscoupling the dual reference
signals being transmitted by the station to the satellite to
compensate for the crosspolarization due to the propagative medium,
including first means for adjusting the amplitude and phase of the
dual reference signals being crosscoupled and transmitted; and
(b) second means for controllably crosscoupling the dual reference
signals being received by the station via the satellite to
compensate for the crosspolarization due to the propagative medium,
including second means for adjusting the amplitude and phase of the
received dual reference signals being corsscoupled; wherein
(c) said first crosscoupling means includes means, coupled to said
second crosscoupling means, for generating output signals
representing a correlation between the crosspolarization produced
by the propagative medium acting on the dual reference signals
transmitted by the station to the satellite and the dual reference
signals received by the station, said first adjusting means being
responsive to the output signals to adjust the amplitude and phase
of the dual reference signals being crosscoupled and transmitted,
said generating means including
(i) memory means, coupled to said second adjusting means, for
storing predetermined pairs of first and second complex variable
signals, each of the first and second complex variable signals
having amplitude and phase information, said second adjusting means
including means for producing control signals representing the
adjustment in amplitude and phase of the received and crosscoupled
dual reference signals, and said memory means having means for
outputting one pair of the complex variable signals in response to
one of the control signals, and
(ii) means, responsive to the one pair of complex variable signals
and the control signals, for producing the output signals.
2. Apparatus for controlling the crosspolarization between first
and second reference signals propagated in a frequency reuse
satellite communications system having a plurality of stations
communicating via a satellite, in which the first and second
reference signals transmitted by each station are of different
polarization, and the propagative medium around the station causes
the crosspolarization of the first and second reference signals
transmitted and received by each station, the apparatus being for
use at each station, comprising:
(a) first means for controllably crosscoupling the first and second
reference signals being transmitted by the station to the satellite
to compensate for the crosspolarization due to the propagative
medium, including means for adjusting the amplitude and phase of
the first and second reference signals being crosscoupled; and
(b) second means for controllably crosscoupling the first and
second reference signals being received by the station via the
satellite to compensate for the crosspolarization due to the
propagative medium; wherein
(c) said first crosscoupling means includes means, responsive to
the first and second reference signals which have been crosscoupled
by said second crosscoupling means, for generating output signals
representing the crosspolarization of the first and second
reference signals transmitted to the satellite, said adjusting
means being responsive to the output signals to adjust the
amplitude and phase of the first and second reference signals being
crosscoupled and transmitted, said generating means including
(i) first means for producing a first pair of signals, one
corresponding to the first reference signal crosscoupled by said
second crosscoupling means and the other corresponding to the first
reference signal crosscoupled into the second reference signal by
said first crosscoupling means; and
(ii) second means, connected to said first producing means, for
producing a second pair of control signals representing the
difference in amplitude and phase between the first pair of
signals, the second pair being the output signals.
3. Apparatus for controlling the crosspolarization between dual
reference signals propagated in a frequency reuse satellite
communications system having a plurality of stations communicating
via a satellite, in which the dual reference signals transmitted by
each station are of a different polarization, and the propagative
medium around each station causes the crosspolarization of the dual
reference signals transmitted and received by each station , the
apparatus being for use at each station, comprising:
(a) first means for controllably crosscoupling in phase and
amplitude the dual reference signals being transmitted by the
station to the satellite to compensate for the corsspolarization
due to the propagative medium;
(b) second means for controllably crosscoupling and for adjusting
in phase and amplitude the dual reference signals being received by
the station via the satellite to compensate for the
crosspolarization due to the propagative medium, including means
for generating first, second, third and fourth control signals
proportional to the adjustment in amplitude and phase of the dual
reference signals, respectively, and
(c) means, responsive to said control signals, for generating
output signals representing a correlation between the
crosspolarization produced by the propagative medium acting on the
dual reference signals transmitted by the station to the satellite
and the dual reference signals received by the station,
including
(i) memory means, responsive to one of said control signals, for
producing a pair of complex variable signals, the pair having a
first amplitude signal, a first phase signal, a second amplitude
signal and a second phase signal; and
(ii) parameter generator means for generating a first output
amplitude signal, a first output phase signal, a second output
amplitude signal and a second output phase signal, including a
first adder for adding the first phase control signal and the
second phase signal of the pair to produce the second output phase
signal, a second adder for adding the second phase control signal
and the first phase signal of the pair to produce the second output
phase signal, a first multiplier to multiply the first amplitude
control signal and the second amplitude signal of the pair to
produce the second output amplitude signal, and a second multiplier
to multiply the second amplitude control signal and the first
amplitude signal of the pair to produce the first output amplitude
signal, wherein said first crosscoupling means is responsive to the
first output signal, the second output signal, the third output
signal and the fourth output signal.
4. Apparatus for controlling the crosspolarization between first
dual signals propagated in a frequency reuse satellite
communications system having a plurality of remote stations and a
local station communicating via a satellite, in which the first
dual signals transmitted by each station are of different
polarization and the propagative medium around the local station
causes the crosspolarization of the first dual signals transmitted
by the remote stations and the local station, the apparatus being
for use at the local station, comprising:
(a) first adjustable means for controllably crosscoupling the first
dual signals transmitted by the local station to the satellite to
compensate for the crosspolarization due to the propagative
medium;
(b) second adjustable means for controllably crosscoupling the
first dual signals received by the local station via the satellite
to compensate for the crosspolarization due to the propagative
medium; and
(c) closed-loop control means for transmitting second
dual-polarized reference signals via said first adjustable means
and through the propagative medium, and for receiving the
transmitted reference signals through the propagative medium and
via said second adjustable means, said second adjustable means
being adjusted in response to the received reference signals to
control the crosscoupling of the received first dual signals and
said first adjustable means being adjusted in response to the
received reference signals to control the crosscoupling of the
transmitted first dual signals.
5. Apparatus according to claim 4 wherein said first adjustable
means comprises first crosscoupling network means for adjusting the
amplitude and phase of the first dual signals and the second
reference signals and wherein said second adjustable means
comprises second crosscoupling network means for adjusting the
amplitude and phase of the first dual signals and the second
reference signals.
6. Apparatus according to claim 5 wherein said closed-loop control
means comprises:
(a) filter and second reference signal detector means for adjusting
said second crosscoupling means in response to the second reference
signals; and
(b) function generator means for adjusting said first crosscoupling
network means in response to the amplitude and phase adjustment of
said second crosscroupling network means.
7. Apparatus for controlling the crosspolarization between first
dual signals propagated in a frequency reuse satellite
communications system having a plurality of remote stations and a
local station communicating via a satellite, in which the first
dual signals transmitted by each station are of different
polarization, and the propagative medium around the local station
causes the crosspolarization of the first dual signals transmitted
by the remote stations and the local station. comprising:
(a) first adjustable means for controllably crosscoupling the first
dual signals transmitted by the local station to the satellite to
compensate for the crosspolarization due to the propagative
medium;
(b) second adjustable means for controllably crosscoupling the
first dual signals received by the local station via the satellite
to compensate for the crosspolarization due to the propagative
medium;
(d) first closed-loop control means, including said first
adjustable means, for transmitting second dual-polarized reference
signals uplink via said first adjustable means through the
propagative medium, and for receiving the second transmitted
reference signals through the propagative medium down-link via said
second adjustable means, said first adjustable means being adjusted
in response to the second reference signals received down-link via
said second adjustable means; and
(e) second closed-loop control means, including said second
adjustable means, for receiving third dual-polarized reference
signals being received down-link through the propagative medium
into said second adjustable means, the third reference signals
having substantially only down-link crosspolarization due to the
propagative medium, said second adjustable means being adjusted in
response to the third reference signals received via said second
adjustable means.
8. Apparatus according to claim 7 wherein said first adjustable
means comprises first crosscoupling network means for adjusting the
amplitude and phase of the first dual signals and the second
reference signals and wherein said second adjustable means
comprises second crosscoupling network means for adjusting the
amplitude and phase of the first dual signals, the second reference
signals and the third reference signals.
9. Apparatus according to claim 8 wherein said first closed-loop
control means comprises function generator means for adjusting said
first crosscoupling network means in response to the second
reference signals adjusted by said second crosscoupling means.
10. Apparatus according to claim 9 wherein said second closed-loop
control means comprises filter and third reference signal detector
means for adjusting said second crosscoupling network means in
response to the third reference signals.
11. Apparatus according to claim 10 wherein said means for
receiving receives the third reference signals from said satellite,
and said satellite also relays the transmitted second reference
signals to said second crosscoupling network means.
12. Apparatus according to claim 11 wherein said function generator
means includes means, responsive to the second reference signals
adjusted by said second crosscoupling network means, for
compensating the first transmitted signals for amplitude and phase
shifts introduced by said satellite.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to communications systems and,
more particularly, to apparatus and methods for controlling
crosspolarization of signals in a frequency reuse system in which
dual polarized signals carry independent information at the same
frequency.
In recent years, the demand for communications has grown
tremendously, and an even more rapid growth is expected in the
future. Both terrestrial communications systems and satellite
communications systems have been improved and expanded to meet this
demand. In the field of satellite communications, for example,
because of its many advantages, even greater demand is placed on
this form of communications. This has resulted in the allocated
spectrum for satellite communications becoming more and more
crowded.
In view of this demand, substantial efforts have been made to try
to utilize the frequency spectrum more efficiently, particularly
for satellite communications systems. One effort has resulted in a
frequency reuse communications technique in which two signals
having independent information share the same channel frequency. In
other words, since two signals share a frequency, every frequency
in the spectrum can be used twice, thereby expanding the capacity
of the communication channels by a factor of two.
One way to achieve a frequency reuse technique is to employ
orthogonal polarizations, that is, dual signals which are
orthagonally polarized in relation to one another. Typically, the
two signals are either linearly polarized, in which the signals are
transmitted at right angles to one another, or are oppositesensed
circularly polarized, in which the two signals rotate in opposite
directions.
The feasability of the frequency reuse technique depends on the
amount of discrimination which can be achieved between the two
signals. For various reasons, during the transmission of the
signals there will always be some amount of signal energy
transferred from one polarization to the other. This energy
transfer is called the crosspolarization effect, which will result
in some level of interference in each of the two signals. The
extent of this effect determines the performance of the
dual-polarization system.
There are many sources in the communications link which will cause
the crosspolarization effect. In transmitting and receiving systems
generally, for example, the antennae, the wave guide, and the
orthomode transducer can cause crosspolarization. In a satellite
communications system, in the propagative medium, the rain, clouds,
snow, etc., can cause crosspolarization. Among all the
crosspolarization effects, rain-crosspolarization at microwave
frequencies has been found to be the most serious problem. This is
owing to the fact that the problems in the transmitting and
receiving systems can be improved by carefully designing these
systems. The effects of clouds and snow are negligible compared to
the effect of rain drops, but the rainfall, of course, can not be
controlled.
Many different systems have been designed to solve the
rain-crosspolarization problem in satellite communications systems.
While these systems are different, their basic approaches are all
the same. Each receiving earth station in the satellite
communications system receiving a transmission of dual-polarized
signals from one transmitting earth station, attempts to cancel the
crosspolarization in the received signals induced by any rain at
both the transmitting and receiving end. More particularly, a
receiving station will have a network which is set or adjusted to
cancel the crosspolarization of signals being received from the one
transmitting station in the system.
A problem with the above cancellation system is that if a receiving
or local station is intended to receive, simultaneously, fifty
dual-polarized signals transmitted by fifty different transmitting
or remote stations located around the world, then fifty such
networks are necessary to cancel, respectively, the
crosspolarization in the fifty dual-polarized signals being
received. This is because it is not unlikely that the rain pattern
at many, if not all, of the transmitting stations, may be different
from one another. The different rain patterns produce different
rain-crosspolarization effects, which means that the dual-polarized
signals being propagated through the rain around one transmitting
station will be crosspolarized differently than the dual-polarized
signals being propagated through the rain around another
transmitting station. Consequently, the receiving station will
require the fifty different networks, each of which will be
adjusted to cancel the crosspolarization of signals being received
from a corresponding transmitting station.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a novel
apparatus and method for cancelling or compensating for the
crosspolarization of signals in a frequency reuse communications
system.
It is another object of the present invention to compensate for or
cancel the crosspolarization of signals in a satellite
communications system utilizing a frequency reuse technique.
A yet other object of the present invention is to provide a
relatively simple polarization compensating or cancelling system
over prior systems in a satellite communications system having a
plurality of earth stations in which a receiving station is able to
receive simultaneously signals from a plurality of transmitting
stations.
The foregoing and other objects of the invention are obtained by
providing apparatus for controlling crosspolarization between
signals propagated in a frequency reuse communications system
having at least one remote station and a local station, the
apparatus being for use at the local station, including first means
for compensating for the crosspolarization of signals transmitted
by the local station due to the propagative medium around the local
station, and second means for compensating for the
crosspolarization of signals received by the local station from the
remote station due to the propagative medium around the local
station.
The foregoing and other objects of the invention are also obtained
by providing a method of controlling the polarization between
signals propagated in a frequency reuse communications system
having a plurality of stations in which the signals transmitted by
each station are of different polarization and the propagative
medium around each station causes the crosspolarization of signals
transmitted and received by the stations, comprising, at each
station, the steps of transmitting dual-polarized reference signals
through the medium, receiving the transmitted reference signals
through the medium, and adjusting the transmitted and received
reference signals to compensate for the crosspolarization of the
reference signals transmitted through the propagative medium and to
compensate for the crosspolarization of the reference signals
received through the propagative medium.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing generally a prior system.
FIG. 2 is a block diagram showing generically the apparatus of the
present invention.
FIG. 3 illustrates in more detail one embodiment of the present
invention.
FIG. 4 shows schematically a function generator used in the
embodiment of FIG. 3.
FIG. 5 illustrates in more detail another embodiment of the present
invention.
DETAILED DESCRIPTION OF THE DRAWINGS
In this specification, the structure, function and operation of the
invention will be described in a manner to allow those skilled in
the art to make and use the invention. However, it is recognized,
in view of the nature of the subject matter of the invention, that
those skilled in the art may want to understand more fully the
mathematical and other principles under which the invention is
designed. Consequently, there is included in this application an
appendix which is attached after the descriptive portion of this
specification and prior to the claims to become a part of this
application. Occasionally, reference will be made throughout this
specification to this appendix. Also, like reference characters are
employed on FIGS. 1-5 of the application drawings and the appendix
for a better understanding of the invention.
In FIG. 1, there is shown a satellite communications system 10 for
carrying on communications between a plurality of earth stations
12, 14 and 16 via a communications satellite 18. Each one of the
earth stations 12, 14 and 16 is capable of transmitting information
to and receiving information from any of the other earth stations
including the one earth station. For purposes of explanation, the
earth station 14 is shown as transmitting information uplink to the
satellite 18 through a propagative medium M.sub.1, with the earth
station 12 receiving this information through a propagative medium
M.sub.2, these links being shown in full lines. The earth station
14 is transmitting two signals x.sub.1, x.sub.2 which carry
independent information and are orthogonally polarized, but are at
the same frequency. As these signals x.sub.1 and x.sub.2 are
transmitted uplink towards the satellite 18, the propagative medium
M.sub.1 will, particularly if it is raining around the station 14,
crosspolarize these signals so that some amount of energy in each
one of the signals x.sub.1 and x.sub.2 will be transferred to the
other of these signals.
The signals x.sub.1 and x.sub.2 are then relayed by the satellite
18 to the earth station 12 through the propagative medium M.sub.2.
During this transmission through the medium M.sub.2, these signals
x.sub.1 and x.sub.2 again will be crosspolarized due to the medium
M.sub.2 around the station 12, particularly if it is raining. Thus,
the station 12 receives not the pure signals x.sub.1 and x.sub.2,
but the crosspolarized signal x.sub.1 coupled with some energy from
x.sub.2, and the crosspolarized signal x.sub.2 coupled with some
energy from x.sub.1. In other words, the received signals are
x.sub.1 plus x.sub.2 at some amplitude and phase, and x.sub.2 plus
x.sub.1 at some amplitude and phase.
An adaptive feedback control system 20, at the earth station 12,
receives the crosspolarized signals x.sub.1 and x.sub.2 and
cancels, or at least substantially compensates for, the
crosspolarization in these respective signals to provide two output
signals y.sub.1 and y.sub.2, corresponding to the pure signals,
respectively, x.sub.1 and x.sub.2 transmitted at the station 14.
The system 20 includes a crosscoupling network K or 22 which
receives the crosspolarized signals x.sub.1 and x.sub.2, filters
24-1 and 24-2 that filter, respectively, reference or pilot signals
f.sub.1 and f.sub.2, to be more fully described, from the network
K, and amplitude and phase detectors 26-1 and 26-2 which respond,
respectively, to the filtered reference signals to control the
network 22.
As illustrated in FIG. 1, the corsscoupling network 22 includes an
adjustable phase shifter 28 and an adjustable attenuator 30 leading
to an adder 31, crosscoupled with an adjustable phase shifter 32
and an adjustable attenuator 34 leading to an adder 36. In the
network 22, the crosspolarized received signal x.sub.1 is fed
through the phase shifter 28 and attenuator 30 to the adder 31
where it is added to the crosspolarized received signal x.sub.2.
The phase shifter 28 and attenuator 30, if set properly, adjusts
the phase and amplitude of the received signal x.sub.1 such that it
will cancel in the adder 31 the amount of energy of the signal
x.sub.1 crosspolarized into received signal x.sub.2, whereby the
output will be signal y.sub.2. Similarly, the received signal
x.sub.2 is fed through the phase shifter 32 and attenuator 34 to
the adder 36 where it is added to the received signal x.sub.1. If
the shifter 32 and attenuator 34 are set properly, the phase and
amplitude of the received signal x.sub.2 will be adjusted such that
in the adder 36, the amount of signal energy of signal x.sub.2 in
the received signal x.sub.1 will be cancelled to provide the output
y.sub.1.
Proper adjustment or setting of the adjustable components 28, 30,
32 and 34 occurs in the following manner. The earth station 14
transmits with the signals x.sub.1 and x.sub.2 the two,
orthogonally polarized, pilot or reference signals f.sub.1 and
f.sub.2 having the same amplitude, but different frequencies from
one another. These pilot signals f.sub.1 and f.sub.2 are
crosspolarized in the same manner as the signals x.sub.1 and
x.sub.2 by the propagative media M.sub.1 and M.sub.2 so as to have
some of their signal energy transferred to each other during
transmission. Before passing completely through the crosscoupling
network 22, the signals x.sub.1, x.sub.2, f.sub.1 and f.sub.2 are
fed to the filters 24-1 and 24-2. Filter 24-1 filters out the pilot
signals f.sub.1 in the paths shown in FIG. 1, respectively, and
sends them to the amplitude and phase detector 26-1. The detector
26-1 then produces two output voltages proportional to the
difference, respectively, in phase and amplitude of the two input
signals f.sub.1. One voltage signal is then used to adjust the
phase shifter 28, while the other voltage signal is used to adjust
the attenuator 30. When the difference in amplitude and phase of
the filtered signals f.sub.1 is 0, then the phase shifter 28 and
attenuator 30, will be set such that the pure signal y.sub.2 can be
obtained.
Similarly, filter 24-2 filters the signals f.sub.2 in the paths
shown in FIG. 1, with the detector 26-2 producing two voltage
signals proportional to the amplitude and phase difference of the
input signals f.sub.2. These voltage signals then adjust the phase
shifter 32 and attenuator 34, and when the difference in amplitude
and phase of the signals f.sub.2 is 0, the shifter 32 and
attenuator 34 will be set such that the pure signal y.sub.1 can be
obtained. Thus, the adaptive feedback control system 20 will be set
to receive and cancel the crosspolarization of signals transmitted
by the station 14 to the station 12.
Different rain conditions will crosspolarize signals differently.
It will be appreciated that the earth station 16 may be
experiencing a different rain condition than the earth station 14.
Consequently, the crosspolarized signals transmitted uplink through
the propagative medium M.sub.1 ' from the station 16 will be
crosspolarized differently than the signals propagated through the
medium M.sub.1 from the station 14. Therefore, if the adaptive
feedback control system 20 is set to cancel the crosspolarization
of signals from the station 14, the system 20 will not be set to
cancel signals received simultaneously from the earth station 16.
If only this one system 20 were at the station 12, the output
signals from the crosscoupling network 22 transmitted by the
station 16 could still have some interference due to the
crosspolarization effect and the inability of the system 20 to
satisfactorily cancel this. There could be another feedback control
system 20 at the station 12 to process the signals received from
the station 16, and additional feedback systems 20 for each other
earth station in the system 10, but this makes the system expensive
and complex.
FIG. 2 will be used to show generically the manner in which the
present invention enables the earth station 12, which may be
considered to be a local station, to receive simultaneously dual or
orthogonally polarized signals from a plurality of other or remote
earth stations and to cancel, or at least substantially compensate
for, the crosspolarization effect with a minimum number of
crosscoupling networks such as network K or 22 shown in FIG. 1.
Before discussing FIG. 2, one particular aspect about satellite
communications systems should be noted. Signals transmitted uplink
to the satellite 18 by an earth station such as station 12, are
transmitted on one frequency carrier at, for example, 14 GHz,
whereas these signals, when transmitted downlink from the satellite
18 to the intended earth station, are received on a different
carrier at, for example, 11 GHz. The significance of these two
different uplink and downlink carriers is that the
crosspolarization due to rain around a given station such as
station 12 will be different on the uplink signals than on the
downlink signals. Thus, with reference to FIG. 2, if dual polarized
signals are transmitted by the station 12 uplink through the
propagative medium, and then these same signals are relayed by the
satellite 18 downlink through the same propagative medium, back to
the station 12, the signals will be crosspolarized differently in
the uplink transmission than in the downlink transmission. The
effect of this different rain-crosspolarization is indicated as
M.sub.1 and M.sub.2 in FIG. 2, which means that the same rain
conditions or medium around the station 12 will produce different
crosspolarizations on the uplink and downlink transmission paths.
It should be noted that whereas in FIG. 1, the terms M.sub.1 and
M.sub.2 related to the rain-crosspolarization at separate earth
stations on the uplink and downlink paths, respectively, in FIG. 2
and the remaining figures, M.sub.1 and M.sub.2 are used to denote
such different rain-crosspolarization around a single earth station
such as station 12 for the uplink and downlink paths.
FIG. 2 illustrates apparatus 38 at a single earth station such as
station 12 for compensating for the crosspolarization effect so as
to enable the receiving of signals simultaneously from earth
station 14, earth station 16 and other earth stations in the
satellite communications system 10, as indicated, as well as from
the station 12. The apparatus 38 includes an adaptive feedback
control system 20' which is substantially the same as the feedback
system 20 shown in FIG. 1. The system 20' includes a crosscoupling
network K.sub.2 or 22', which is substantially the same as the
network 22. Also included in the system 20' are the filters 24-1,
24-2 and detectors 26-1, 26-2 which are used to control the network
22' in the same manner that network 22 is controlled.
The apparatus 38 also includes a second crosscoupling network
K.sub.1 of 40 which is substantially the same as the crosscoupling
network 22'. A function generator 42 responds to various input
signals to produce output signals for controlling the crosscoupling
network 40. Both embodiments of the present invention to be
described employ the adaptive feedback control system 20', the
crosscoupling network 40 and the function generator 42. In one
embodiment, the network 40 is controlled directly by the network
22' through the function generator 42. That is, signals
corresponding to the setting of the phase shifters and attenuators
in the network 22' are fed to the function generator 42, which
responds to these signals by generating output signals to control
the network 40. Thus, the setting of the phase shifters and
attenuators in the network 40 are locked to or follow the setting
of the shifters and attenuators in the network 22'. In the other
embodiment, a circuit 44 is included in the apparatus 38, to
receive, as input information, signals from the output of the
feedback system 20' and signals being crosscoupled in the network
40. In this other embodiment, the function generator 42 provides
output signals to control the network 40 in response to output
signals from the circuit 44, and not from the signals mentioned
above corresponding to the settings in the network 22'.
The apparatus 38 is used to cancel, or at least substantially
compensate for, the rain crosspolarization effects occurring as a
result of the weather conditions around the station 12. The network
40 is controllable to compensate the signals to be transmitted by
the station 12 for the uplink rain-crosspolarization, while the
network 22' is controllable to compensate for the
rain-crosspolarization of all incoming signals from the other
stations as well as the station 12 due to the downlink
transmission. This will enable simultaneous reception and
crosspolarization cancellation or compensation by the apparatus 38
in the station 12 in the following manner.
Each earth station in the satellite communications system 10,
including the stations 12, 14 and 16, will have the apparatus 38.
This means that the signals being transmitted uplink from any of
the earth stations 12, 14 and 16 will arrive at the satellite 18
without being crosspolarized due to their respective local rain
conditions since each crosscoupling network 40 will compensate for
such rain-crosspolarizations. Then, when the uplink signals are
relayed by the satellite 18 downlink to the receiving station 12,
all of these signals from the various stations will be
rain-crosspolarized in the same manner since they all pass through
the same rain conditions around the station 12. Therefore, since
the crosscoupling network 22' compensates for the downlink
crosspolarization occurring due to this rain condition at the earth
station 12, all of the incoming signals will be compensated.
FIG. 3 illustrates one embodiment or apparatus 38' of the apparatus
38 shown in FIG. 2. The apparatus 38' includes the adaptive
feedback control system 20', the corresponding network 40 and the
function generator 42, all of which are at a single earth station
such as local station 12. In order to explain this embodiment, the
apparatus 38' is shown as transmitting the dual-polarized signals
x.sub.1 and x.sub.2 along with the dual-polarized pilot or
reference signals f.sub.1 and f.sub.2 through the crosscoupling
network 40 and then via an uplink path through medium M.sub.1 to
the satellite 18. The apparatus 38' then receives its own
transmission from the satellite 18 via the downlink path through
the same medium M.sub.2 and crosscouples the signals through the
adaptive feedback system 20' to obtain the pure signals of y.sub.1,
y.sub.2, f.sub.1 and f.sub.2. The purpose of this transmission and
reception, as will be further described, is to use signals f.sub.1
and f.sub.2 to adjust the crosscoupling networks 40 and 22' to
compensate for the crosspolarization on the propagated signals
occurring, respectively, during the uplink and downlink
transmission. Actually, the station 12 need only receive the pilot
signals f.sub.1 and f.sub.2 to so adjust the crosscoupling networks
40 and 22', as will become apparent. Once the crosscoupling
networks 40 and 22' are set to cancel, or at least substantially
compensate for, this crosspolarization, and assuming all other
remote earth stations in the satellite communications system 10
have set their apparatus 38', then the station 12 can receive
simultaneously transmissions from all such other earth stations.
These transmissions can be processed simultaneously through the
crosscoupling network 22' to cancel the downlink crosspolarization
effect due to the rain conditions around the station 12.
As described in the Appendix, for the same earth station such as
station 12, the crosspolarization due to M.sub.1, on the uplink
path, and the crosspolarization on the downlink due to M.sub.2,
will be highly correlated. That is, since the crosspolarization of
the signals transmitted uplink will be different than the
crosspolarization of the signals transmitted downlink due primarily
to the different carrier frequencies, M.sub.1 is a known function
of M.sub.2. Mathematically, as given in the Appendix, this
relationship is as follows:
Since the crosscoupling network 40 or K.sub.1 is to compensate for
the uplink crosspolarization M.sub.1, and the crosscoupling network
22' or K.sub.2 is to compensate for the downlink crosspolarization
M.sub.2, then the adjustment for the crosscoupling network 40 will
be a known function of the setting of the crosscoupling network
22'. Mathematically, this is written as follows:
Consequently, the generator 42 is a function generator which
controls the adjustment of the crosscoupling network 40 as a
function of the adjustment of the crosscoupling network 22'. The
feasability of the technique to be described in relation to FIG. 3
depends on the possibility of obtaining the correlation function f
in equation 1, and then designing the function generator 42, i.e.,
the function g, to follow any variation in the correlation function
f. It is in fact possible to obtain this correlation function f and
the details are described in the Appendix.
To simplify the description, FIG. 3 illustrates only one half of
the crosscoupling networks 40 and 22'. These halves are used to
purify the signal x.sub.2 so that a pure signal y.sub.2 is obtained
at the output of the adaptive feedback control system 20'. The
other half of the networks 40 and 22' will be the same, and it will
be appreciated by those skilled in the art that the signal x.sub.1
can be purified to obtain y.sub.1 in a similar manner.
The one half of the crosscoupling network 40 for purifying the
signal y.sub.2 includes an adjustable phase shifter 46 and an
adjustable attenuator 48, the output of which is coupled to an
adder 50. Also shown is an adder 52 in the crosscoupling network
40. The function generator 42 includes a memory 54 and an uplink
parameter generator 56. The parameter generator 56 has four inputs
labeled a.sub.1, a.sub.2, .zeta..sub.1 and .zeta..sub.2. These
inputs constitute voltage signals which represent, respectively,
the parameter or setting of the shifters and attenuators in the
crosscoupling network 22'. Thus, a.sub.2 constitutes a voltage
signal representing the setting of the attenuator 30, while
.zeta..sub.2 is a voltage signal representing the setting of the
phase shifter 28.
The function g in equation 2, as noted in Appendix 1, can be
specified by two complex variables g.sub.1 and g.sub.2. The
parameter generator 56, therefore, receives four additional input
voltage signals from the memory 54. Two of these input signals
represent the function g.sub.1 and are shown as .vertline.g.sub.1
.vertline. and g.sub.1 (.phi.) corresponding to the amplitude and
phase of g.sub.1. The other two input signals represent the complex
variable g.sub.2 and are similarly shown as .vertline.g.sub.2
.vertline. and g.sub.2 (.phi.) representing the amplitude and phase
of g.sub.2.
The functions g.sub.1 and g.sub.2 are variable since they depend
not only on the carrier frequencies, but also, on the intensity of
a rainstorm around the station 12. The memory 54, therefore, stores
a plurality of values for g.sub.1 and g.sub.2 corresponding to
various intensities of rainstorms that may be expected in the area
of the earth station 12. These values for g.sub.1 and g.sub.2 can
be calculated in advance and then stored in the memory 54. The fact
that the variables g.sub.1 and g.sub.2 are dependent on rain
intensity, implies that the memory 54 must receive information as
to the rain intensity during operation of the apparatus 38' to
output the correct values of the variables g.sub.1 and g.sub.2. As
described in the Appendix, any of the parameters a.sub.1, a.sub.2,
.zeta..sub.1 or .zeta..sub.2 can be used as an indicator of rain
rate in determining the values of the functions g.sub.1 and
g.sub.2. FIG. 3 illustrates the use of the parameter a.sub.1 which
is fed as the input to the memory 54, whose output will then be the
four voltage signals identifying g.sub.1 and g.sub.2 having values
corresponding to the rain intensity.
The parameter generator 56 receives the eight input voltage signals
indicated in FIG. 3, combines them in a particular way to be
described in connection with FIG. 4, and then outputs four voltage
signals a.sub.1 ', a.sub.2 ', .zeta.'.sub.1 ' and .zeta.'.sub.2 '
which control, respectively, the parameters or settings of the
attenuators and phase shifters in the network 40. The attenuator 48
is thus regulated by one voltage signal a.sub.2 ' to change its
parameter or setting and the phase shifter 46 is adjusted by
another voltage signal .zeta.'.sub.2 to have its setting varied in
accordance with this voltage signal.
In the operation of the apparatus 38', assume that, while
communicating with another station, the intensity of a rainstorm in
the area surrounding the station 12 increases and that, therefore,
the crosscoupling networks 40 and 22' have to be reset to cancel
the new crosspolarization effects. The station 12 will continue
transmitting the dual polarized pilot signals f.sub.1 and f.sub.2
to the satellite 18, and receive back these pilot signals f.sub.1
and f.sub.2.
During the transmission of the pilot signal f.sub.1, uplink from
the network 40 and then downlink to the feedback control system
20', the crosspolarization effect will result in some signal energy
of the pilot signal f.sub.1 being transferred into the pilot signal
f.sub.2. The received pilot signal f.sub.1 is then fed through the
phase shifter 28 and attenuator 30 to the adder 32, while the
received pilot signal f.sub.2, which now has some signal energy of
the pilot signal f.sub.1 crosspolarized into it, is also fed to the
adder 32, as shown in FIG. 3. The filter 24-1, which has one input
coupled to the output of the attenuator 30 and another input
coupled to receive such received pilot signal f.sub.2, filters the
pilot signals f.sub.1 in these two input signals and feeds them to
the detector 26-1.
At this time, the two pilot signals f.sub.1 will have a difference
in amplitude and phase. The detector 26-1 then provides two output
voltage signals proportional, respectively, to the difference in
phase and amplitude between the input signals f.sub.1. One voltage
signal then adjusts the phase shifter 28 and the other voltage
signal adjusts the attenuator 30.
The process of adjusting the shifter 28 and attenuator 30 continues
until the amplitude and phase of the two pilot signals f.sub.1 fed
into the detector 26-1 are the same. When there is no such
difference in the amplitude and phase of the pilot signals f.sub.1
received by the detector 26-1, this indicates that any interference
due to some signal energy of signal f.sub.1 in the signal f.sub.2
received by the system 20' is cancelled or at least substantially
compensated for. That is, the phase shifter 28 and attenuator 30
will be set such that the amount of signal energy of the signal
f.sub.1 crosspolarized into the signal f.sub.2 will be cancelled in
the adder 32 by the signal whose amplitude and phase has been
adjusted by the shifter 28 and attenuator 30. Thus, the system 20'
will compensate for the round trip cross-polarization effects
produced in the uplink by M.sub.1 (and network K.sub.1) and the
downlink by M.sub.2.
For the reasons given above, the signals entering the satellite 18
from the station 12 should not be cross-polarized. This will not be
the case if the crosscoupling network 22' is only adjusted as
described above; therefore, this is the reason for controlling the
network 40 so that it introduces some crosspolarization into
signals f.sub.1 and f.sub.2 before transmitting these signals,
which crosspolarization will then be cancelled by the propagative
medium M.sub.1 during transmission to the satellite 18. With the
network 22' set as indicated above when the detector 26-1 senses no
difference in amplitude and phase between the input pilot signals
f.sub.1, the parameters of the shifter 28 and attenuator 30 are
properly set for controlling the network 40. The function generator
42 thus responds to these parameters of the network 22' to adjust
the phase shifter 46 and attenuator 48. Consequently, some energy
of the signal f.sub.1 is crosscoupled through the phase shifter 46
and attenuator 48 into the signal f.sub.2 via the adder 50.
Thereafter, when this signal f.sub.2 with the crosscoupled signal
f.sub.1 is transmitted uplink to the satellite 18, the latter
signal f.sub.1 will be cancelled due to the crosspolarization
effect by M.sub.1. With the networks 22' and 40 thus adjusted, and
assuming each earth station in the system 10 has so adjusted its
networks 22' and 40, it will be appreciated that the earth station
12 can now receive simultaneously dual polarized signals x.sub.1
and x.sub.2 from every earth station and purify these signals to
provide the output signals y.sub.1 and y.sub.2. It will be seen
that in practice, since network 40 is directly controlled by
network 22' through generator 42, during the resetting operation
each of these networks is simultaneously being reset until the
adjustment is made for cancelling the uplink and downlink
crosspolarization.
It also can be seen that the apparatus 38' constitutes a
closed-loop control system for adjusting the networks 40 or K.sub.1
and 22' or K.sub.2. Closed-loop control of network 40 is produced
by sending the signals f.sub.1, f.sub.2 through the network 40 to
the satellite 18 and receiving the signals f.sub.1, f.sub.2 from
the satellite 18 and via the network 22' for coupling to, for
example filter, 24-1 and detector 26-1. The output of detector 26-1
essentially is coupled to function generator 42 whose output then
controls network K.sub.1. This overall path constitutes a closed
loop path for control of network 40. Also, the output of detector
26-1 is used to control the network 22' in response to signal
f.sub.1, whereby a closed loop path is provided for network
22'.
FIG. 4 illustrates in more detail the uplink parameter generator
56. This generator 56 includes two adders 58 and 60 and two
multipliers 62 and 64. The adder 58 adds the voltage signals
.zeta..sub.1 and g.sub.2 (.phi.) to produce the voltage signal
corresponding to the parameter .zeta.'.sub.2 '. The adder 60 adds
the voltage signals .zeta..sub.2 and g.sub.1 (.phi.) to produce the
voltage signal .zeta.'.sub.1. The multiplier 62 multiplies the
signal a.sub.1 and .vertline.g.sub.2 .vertline. to produce the
signal a.sub.2 '. The multiplier 64 multiplies the signals
.vertline.g.sub.1 .vertline. and a.sub.2 to produce the signal
a.sub.1 '. The generator 56 as well as the memory 54 can be
implemented digitally if the satellite communication system 10
constitutes a digital transmission system, or analog.
In the technique used with the apparatus 38' discussed above, each
earth station transmits its own pilot signals and receives them
back to compensate the uplink and downlink crosspolarization
effects, separately. The received pilot signals always contain the
combined effect of both the uplink and downlink crosspolarizations.
Therefore, the correlation function f has to be known, and,
accordingly, the correlation function g to obtain the separate
information about the uplink and downlink crosspolarizations,
respectively. However, if information can be obtained as to the
downlink crosspolarization only, then the correlation technique
would not be required at all because the uplink crosspolarization
can be deduced from this separate downlink crosspolarization
information and the pilot signals transmitted and received by the
earth station 12. A technique in which the correlation functions
are not required will be described in relation to FIG. 5 which
illustrates an alternative apparatus 38".
FIG. 5 shows the adaptive feedback control system 20' which
receives various signals from the satellite 18 and includes the
crosscoupling network 22', the filter 24-1 and the detector 26-1.
The network 22' includes the adjustable phase shifter 28 and
adjustable attenuator 30, which are controlled by the output
voltage signals from the detector 26-1. As with the FIG. 3
embodiment, the FIG. 5 embodiment will be discussed only in terms
of obtaining a purified signal y.sub.2, it being appreciated that
those skilled in the art would then know how to obtain a purified
signal y.sub.1. Therefore, only one half of the crosscoupling
networks 22' and 40 are shown in FIG. 5.
As may be seen by comparing FIGS. 3 and 5, the crosscoupling
network 40 is not controlled directly by the crosscoupling network
20' in FIG. 5 as it is in FIG. 3. That is, the parameters or
settings of the phase shifter 28 and attenuator 30 are not employed
via the memory 54 and parameter generator 56 to adjust the settings
for the phase shifter 46 and attenuator 48. Rather, the circuit 44
(see also FIG. 2) receives input signals from the output of the
nework 22' over a path 68 and input signals from the network 40
over paths 70 and 72, and provides output signals over paths 74 and
76 to an amplitude and phase detector 78. The detector 78
constitutes the function generator 42 shown in FIG. 2 and provides
output voltage signals over paths 80 and 82 in response to any
difference in phase and amplitude between its input signals to
control, respectively, the phase shifter 46 and attenuator 48.
The circuit 44 includes a satellite compensation network 84 having
an adjustable phase shifter 86 receiving the signals on the path 68
and an variable gain amplifier 88 which receives the phase shifted
signals from the phase shifter 86 and provides attenuated output
signals over a line 90 to a filter 92 as one input. The other input
to the filter 92 is the signal on path 70. The filter 92 thus
provides a pair of output signals on lines 94 and 96 to an
amplitude and phase detector 98. In response to any difference in
phase and amplitude between the signals on lines 94 and 96, the
detector 98 provides voltage signals on lines 100 and 102 to adjust
the phase shifter 86 and amplifier 88 in a similar manner that
detectors 26-1 and 78 control the phase shifters and attenuators in
networks 22' and 40.
Another filter 104 receives the output signals on line 90 from the
satellite compensation network 84 as one input and the signals on
line 72 as the other input. The filter 104 provides the two output
signals on the lines 74 and 76 to the detector 78.
The apparatus 38" shown in FIG. 5 will operate in the following
manner. Dual polarized pilot signals f.sub.1 and f.sub.2 are
transmitted from the satellite 18 downlink and into the
crosscoupling network 22' of the feedback system 20'. These pilot
signals f.sub.1 and f.sub.2 will be crosspolarized only as a result
of the crosspolarization produced by M.sub.2. In other words, the
pilot signals f.sub.1 and f.sub.2 will have only the downlink
crosspolarization effect. This can be accomplished in either of two
ways. One way is to place an oscillator (not shown) on the
satellite 18, which will then generate the signals f.sub.1 and
f.sub.2 and broadcast them to all the earth stations in the system.
Another way is to have the signals f.sub.1 and f.sub.2 transmitted
to the satellite 18 by an earth station which is located in an area
where it never rains or rains only slightly such that the
rain-crosspolarization effect is negligible, such as an earth
station located in a desert area. The signals f.sub.1 and f.sub.2
thus transmitted by this earth station to the satellite 18 will
experience none or negligible rain-crosspolarization in the uplink
path, and when these signals are then relayed downlink by the
satellite 18 they will contain only the downlink
rain-crosspolarization.
The crosscoupling network 22' thus receives the rain-crosspolarized
pilot signals f.sub.1 and f.sub.2 as shown in FIG. 5. The received
signal f.sub.1, after being phase shifted by the phase shifter 28
and attenuated by the attenuator 30, is fed as one input to the
filter 24-1, while the received signal f.sub.2, which has some
energy of the signal f.sub.1 crosspolarized into it, is fed as the
other input to the filter 24-1. The filter 24-1, which filters the
signals f.sub.1, therefore provides two output signals f.sub.1 to
the detector 26-1. The detector 26-1 then provides two voltage
signals which are proportional, respectively, to the difference in
phase and amplitude of the signals f.sub.1 to adjust the phase
shifter 28 and attenuator 30, respectively. When the detector 26-1
detects no difference in amplitude and phase between the two input
signals f.sub.1, the phase shifter 28 and attenuator 30 will be set
such that the signal f.sub.1 fed into the added 32 from the
attenuator 30 will cancel the signal f.sub.1 crosspolarized into
the signal f.sub.2. Consequently, the network 22' will now be
adjusted to cancel the downlink crosspolarization effect.
With the network 22' thus adjusted, the network 40 can now be
adjusted to cancel the uplink crosspolarization effect. To do this,
the earth station 12 transmits dual polarized pilot signals f.sub.1
' and f.sub.2 ' uplink to the satellite 18, which then relays these
signals f.sub.1 ' and f.sub.2 ' downlink to the network 20'. The
pilot signals f.sub.1 ' and f.sub.2 ' are at a different frequency
from one another and from the signals f.sub.1 and f.sub.2. These
pilot signals f.sub.1 ', and f.sub.2 ' thus will have the combined
effect of the uplink and downlink crosspolarizations; however,
since the network 22' will cancel the effect of the downlink
crosspolarization, the signal f.sub.2 ' being output by the network
22' from the adder 32 will have some crosspolarized energy of
f.sub.1 ' in it due only to the uplink rain-crosspolarization.
Disregarding the satellite compensation network 84 for the moment,
the filter 104 receives the signal f.sub.2 ' from the lines 68 and
90, and the signal f.sub.1 ' from the output of the attenuator 48
on line 72. The filter 104, which then filters the two pilot
signals f.sub.1 ' on lines 90 and 72, thus provides these two pilot
signals to the detector 78, with one pilot signal f.sub.1 ' having
been relayed through the satellite 18 and having the uplink
crosspolarization information, and the other pilot signal f.sub.1 '
taken directly from the network 40.
The detector 78 then provides output voltage signals on lines 82
and 80, respectively, proportional to any difference in amplitude
and phase between its input signals f.sub.1 ' to adjust the
attenuator 48 and shifter 46. When no such difference occurs, the
attenuator 48 and shifter 46 will be set such that an amount of
signal energy of signal f.sub.1 ' will be included in f.sub.2 ' in
the adder 50 whereby this signal energy will be cancelled due to
the uplink rain-crosspolarization M.sub.1 when transmitted to the
satellite 18.
It will therefore be appreciated that once the networks 40 and 22'
are so adjusted, the signals x.sub.1 and x.sub.2 will arrive at the
satellite 18 with no rain-crosspolarization due to M.sub.1, and
that the network 22' will cancel the downlink
rain-crosspolarization due to M.sub.2 to provide purified signals
y.sub.1 and y.sub.2. Of course, as already indicated the signals
x.sub.1 and x.sub.2 will be coming primarily from remote stations
transmitting to the local stations, rather than from the local
station as shown in FIG. 5.
The two output signals f.sub.1 ' fed into the detector 78 can't be
compared directly without one being processed through network 84
because this one signal will have been transmitted through the
satellite 18 while the other will not be. The satellite 18 itself
and media M.sub.1 and M.sub.2 will introduce an amplitude and phase
difference between these two signals which should be taken into
consideration. Note that this difference is not due to the
crosspolarization effect, but rather to the fact that the satellite
18 and media M.sub.1, M.sub.2 will affect the amplitude and phase
of the pure signal f.sub.1 ' fed into adder 52 and then transmitted
uplink. Thus, the function of the satellite compensation network 84
is to compensate for this difference, and it operates in the
following manner.
The filter 92 filters the pure signal f.sub.2 ' on line 70 and the
pilot signal f.sub.2 ' on line 90 and provides these two signals to
the detector 98. The detector 98 then provides voltage signals,
respectively, on lines 100 and 102 proportional to any difference
in phase and amplitude between its input signals f.sub.2 ' to
adjust the amplifier 88 and shifter 86. When this difference is 0,
the shifter 86 and attenuator 88 will be set to cancel, or at least
substantially compensate for, any such difference in amplitude and
phase shift provided by the satellite 18 and media M.sub.1,
M.sub.2. The reason for using the pilot signals f.sub.2 ' to
compensate for the amplitude and phase shifts introduced by the
satellite 18 and media M.sub.1, M.sub.2 is that the only available
reference for satellite compensation purposes is the pilot signal
f.sub.2 '. Once this compensation occurs, the signal f.sub.1 ' fed
into the filter 104 will be compensated for the amplitude and phase
shift introduced by the satellite 18 and media M.sub.1,
M.sub.2.
As with apparatus 38', the apparatus 38" constitutes a closed-loop
control system for adjusting the networks 40 or K, and 22' or
K.sub.2. Closed loop control of network 22' is produced by
receiving signals f.sub.1, f.sub.2 from the satellite 18 and via
the network 22' for coupling to, for example, filter 24-1 and
detector 26-1. The output of detector 26-1 is used to control the
network 22' in response to signal f.sub.1, whereby a closed-loop
path for network 22' is provided.
Closed-loop control of the network 40 is produced by sending
signals f.sub.1 ', f.sub.2 ' through the network 40 to the
satellite 18 and receiving the signals f.sub.1', f.sub.2 ' from the
satellite 18 and via the network 22'. The output of network 22',
for example signal f.sub.1 ' is then fed via circuit 44 and
detector 78 to control the network 40. It will now be appreciated
that while FIGS. 2-5 illustrate apparatus and methods for setting
the networks K.sub.1 and K.sub.2, in a broad aspect, the invention
contemplates merely the use of these two or equivalent networks,
however they may be adjusted, provided they compensate for the
crosspolarization effect as already described. Furthermore, while
FIGS. 2-5 illustrate the control of network K.sub.1 with network
K.sub.2, as given in Appendix 1, the network K.sub.2 could be
controlled with network K.sub.1.
While the invention has been particularly shown and described with
reference to preferred embodiments thereof, it will be understood
by those skilled in the art that the foregoing and other changes in
form and details may be made therein without departing from the
spirit and scope of the invention.
APPENDIX
Derivation of g
Let the axis of symmetry of an oblate raindrop be oriented with
respect to the horizontal direction at an angle, .theta., called
the "canting angle," and with respect to the signal propagation
direction at an angle, .gamma.. Let x.sub.1, x.sub.2 be the
transmitted signals, y.sub.1, y.sub.2 be the received signals
propagated through the raindrops. If .gamma.=90.degree., which is
the case for horizontal propagation, the relationship between
x.sub.1, x.sub.2 and y.sub.1, y.sub.2 can be represented by the
following expression. ##EQU1## where
and where .alpha..sub.1, .alpha..sub.2, .beta..sub.1, .beta..sub.2
are the attenuation and phase shift constants for signals polarized
in the directions of major and minor axes of the raindrops; L is
the pathlength; .epsilon. and .theta..sub.eff are two empirical
parameters characterizing the random variation of the raindrop
canting angles. The diagonal elements, A, B, in the matrix, M,
represent the wanted signals. The off-diagonal elements,
.epsilon..delta., represent the crosspolarization. This model holds
true for the oblique path of satellite communication in which
.gamma..noteq.90.degree. if T.sub.1, T.sub.2 can be slightly
modified.
Assume K is a network cross-coupling the two received signals with
adjusted amplitude, a.sub.1, a.sub.2 and phase .zeta..sub.1,
.zeta..sub.2, to cancel the crosspolarization. The variables
a.sub.1, a.sub.2, .zeta..sub.1, .zeta..sub.2, are controlled by a
feedback system which detects the pilot signals contained in the
received signal as the reference of adjustment. The effect of the
network, K, can always be represented by a matrix multiplication
and can always be simplified into the form ##EQU2## and the final
received signal after the network is
where M.sub.1, M.sub.2 are the rain crosspolarization matrices for
uplink and downlink, respectively. The system is to adjust K
according to M.sub.1, M.sub.2 such that
The off-diagonal elements are cancelled to be zero; therefore,
there will not be any crosspolarization. However, if there is a
third ground station transmitting signals with uplink rain
crosspolarization matrix, M.sub.1 '.noteq.M.sub.1 then
in general. Therefore, this system cannot work for a second signal
coming from a third ground station.
In the new approach of the present invention every ground station
uses two cross-coupling networks instead of one, K.sub.1 and
K.sub.2, before transmitting and after receiving, to compensate itw
own uplink and downlink rain crosspolarization matrices M.sub.1 and
M.sub.2 *, respectively; i.e., to have
If Eqs. (11) and (12) are satisfied, any signal arriving at the
satellite from any ground station will not be crosspolarized since
every M.sub.1 is compensated by its K.sub.1, and these signals can
be received by any ground station without crosspolarization since
every M.sub.2 is compensated by its K.sub.2.
In order to properly control the networks K.sub.1 and K.sub.2,
every ground station should transmit its own pilot signal to the
satellite and receive it back from the satellite as the reference
to give information about M.sub.1 and M.sub.2. If the time delay
can be neglected due to the slow time variation of
crosspolarization phenomenon, M.sub.1 and M.sub.2 should have every
parameter exactly the same except frequency since the uplink and
downlink paths are the same, but with different frequencies. In
other words, M.sub.1 should be a known function of M.sub.2,
but K.sub.1 and K.sub.2 are to compensate M.sub.1 and M.sub.2,
respectively. K.sub.1 thus must be a known function of K.sub.2,
It will be shown that we can use exactly the same feedback system
currently designed by many people to control K.sub.2, and then
control K.sub.1 by K.sub.2 directly through a known function, g,
without any further feedback loop. The K.sub.2 system controlled by
feedback will cancel the crosspolarization anyway, thus
guaranteeing the round trip matrix to be diagonal.
It will be shown that Eq. (15) will imply Eqs. (11), (12)
automatically if we properly design the g function to follow the
variation of the f function, and the system thus works.
In the old scheme each station corrected incoming signals from only
one station. In the new scheme, each station is to correct its own
outgoing signals for the expected uplink cross-polarization and
correct its own pilot signal plus all incoming signals from other
stations for downlink crosspolarization. It can be shown that two
compensating networks per station, as proposed here, is the minimum
number required in the general multiple-uplink case.
V. System Description
Let ##EQU3## Let the function, f, in Eq. (13) be specified by three
complex variables, f.sub.1, f.sub.2, f.sub.3 as follows:
All of them are supposed to be known variables because f is a known
function. Let the function, g, in Eq. (14) be specified by two
complex variables, g.sub.1, g.sub.2 as follows:
Note that g.sub.1, g.sub.2 are defined as the "cross-ratio" instead
of the "direct ratio" of the off-diagonal elements in K.sub.1 and
K.sub.2. The reason for this will be clear very soon. So long as we
know g.sub.1 and g.sub.2, we can obtain the parameters in K.sub.1
directly from knowledge of K.sub.2.
Some algebra will show that Eq. (11) requires
and Eq. (12) requires
We find that the right hand sides of Eqs. (25) and (28) are of
exactly the same form. For this reason we define g.sub.1 to be the
ratio of them in Eq. (23). Similarly, we define the ratio of Eqs.
(26) to (27) as g.sub.2 in Eq. (24) because their right hand sides
are of the same form. Substituting the ratio in Eqs. (20) through
(22), we have the following simple expressions:
Note that the "cross-ratio" definition of g.sub.1, g.sub.2 in Eqs.
(23), (24) simplifies the expression in Eqs. (29), (30). This is
actually due to the reverse order of matrices in Eqs. (11), (12);
i.e., K.sub.1 is applied before the effect of M.sub.1 but K.sub.2
is applied after the effect of M.sub.2. Since the variables,
f.sub.1, f.sub.2, f.sub.3, are supposed to be known, g.sub.1,
g.sub.2 are therefore known, and we can control K.sub.1 directly
from knowledge of K.sub.2. K.sub.2 is controlled by the currently
desiged feedback system; the whole system thus works.
We next have to check if the g.sub.1, g.sub.2 in Eqs. (29), (30) do
imply that Eqs. (11), (12) can be satisfied. Let the matrix,
K.sub.2, be ##EQU4## where x, y are the phase and amplitude
adjustment controlled by the feedback loop. K.sub.1 will then be,
if controlled by the above scheme, ##EQU5## and the round trip
matrix is ##EQU6## The feedback K.sub.2 system will guarantee this
matrix to be diagonal,
It can be shown that Eqs. (16), (17), (31).about.(35) have a unique
solution for x, y, which is
These are exactly Eqs. (27) and (28), therefore Eqs. (25), (26) and
thus Eqs. (11), (12) will be satisfied and the system does
work.
The control system thus adjusts K.sub.2, and through the g
function, K.sub.1 to make the total round trip signal
uncrosspolarized. By the correct choice of the g function, this
also results in the uplink and downlink being uncrosspolarized
separately.
A critical issue is that the function g, Eqs. (29) and (30), is
varying. It depends on not just the path and frequency but also on
the intensity of the rainstorm. The function relating K.sub.1 and
K.sub.2 must therefore be constantly modified to reflect changing
conditions. The g function can be derived from local information on
the rain storm or more practically, from measurement of K.sub.2
itself. That is, the adjusted value of K.sub.2, which gives
diagonal to the round trip matrix, can be used as a measure of the
rain intensity in defining the function g. As long as K.sub.2 is
monotonic function of the rainstorm, this procedure will not
introduce ambiguities in the final solution.
* * * * *